little correction, tie down the branches that are getting light so the bottom branches are at the same leel as the top ones. you can train any time and as much as you want. it is done as the plant is growing though so if you already have an establish sturdy stalk it is going to be hard to bend over. There are multiple ways to lst. you can bend over the top branch as soon as you can get it to make a full "U" without snapping the stalk. after a week or 2 tied in this position you will see the side growth go crazy (also trainable).
you can combine it with topping. i like to top to an 8 inch tall plant, let veg for another week or two then flower. then i Lst a lot in the following 2 weeks when there is a lot of stretching. as 1 branch takes the lead i tie it down. keeping the best 6-8 shoots. SCROGing really is another form of lst.
In any event the entire purpose of training the plant is to get more buds in a higher intensity area of light. keep that in mind when your bending the plant around...how can i get more bud sites as close to the light as possible?
